Enrolled subjects will include adults with cystic fibrosis, over the age of 18 years, who are at baseline health and colonized with Pseudomonas aeruginosa, and who are newly prescribed Cayston (aztreonam lysine) for inhalation, or who are resuming use of this drug after 28 days off of inhaled antimicrobial agents with activity against Pseudomonas aeruginosa.